Well, it might've happened that way

There are of course other reasons possible too! Often it is the case in fiction or occult beliefs that either humans literally lack the anatomical ability to speak demonic language (or sometimes understand it-) or that demonic languages are so profane that to simply hear them would cause horrible side effects or madness or destruction. It is said in various places in Religious cannon that a mortal cannot bear the true and unfiltered voice or visage of an angelic being or god- or their visage. In Greek myth- the gods were by and large not human. Zeus literally was a living lightening bolt of some sort- and a woman once saw him not disguised and was destroyed by just seeing him. Angels are often described and depicted as 4th dimensional shapes or even things like flaming wheels with many arms and animal faces and the like.

So- there’s that aspect. The cult might speak Latin because they have the translations to speak to demons and do their magic- in Latin- because few works from previous periods survive and few people speak those languages- and well- loads of texts from antiquity were copied into Latin- and it might be impractical or even dangerous or impossible to speak in a demonic tongue or understand it.

The demons- well.... in fiction we often see them speaking vulgate- plain languages of various modern settings and the “common man,” but we see them speak latin too. Why...? Let’s ignore the whole “words might kill or destroy reality” part. A summoned or bound demon may be FORCED to speak latin. It’s a VERY precise language. If you’re dealing with a double crossing lying demon that will use any imprecision to get you (think like the genie that when you wish you could fly turns you into a house fly because you didn’t say you wanted to fly as a human etc.) So speaking to them in Latin can really help that precision.

Now- latin is also the language that educated folks and priests through much of history and the sort of “peak” of demonology and occult origins in the West spoke. So it would make sense that when seducing rulers and clergy and powerful folks- demons may need Latin. In some cases- perhaps they haven’t kept up on earth and were away a long time- and the last time they were here people spoke latin- so of they want to communicate they’ll try to use the last known language anyone understood and they also knew.

And that becomes a two way street doesn’t it? If you want to summon a demon- you’ll probably find a book written in Latin. So you’ll summon the demon in latin. Now- it’s possible the demon just speaks whatever language it was summoned in- but even if not- if someone summons you in latin- and you know latin- you’ll probably speak latin. And if a demon you summoned using latin from a book written in latin, starts speaking latin- and you’re part of a demonic cult and speak latin- you’ll probably just run with latin. As far as keeping things secret in 2020 goes- not a whole lot of folks you’ll run in to speak latin.

And seeing a bunch of books in latin or dudes in robes chanting latin- either they’re monks or priests or something, a Shakespeare club maybe? Or they are just some “whackos” who “think” they can summon demons right? So overall- it would make sense. Latin is uncommon but it isn’t exactly exotic.

So if we examine it- it actually isn’t so odd that demons and demonic cults are often seen speaking latin. If. Demon DOES at least have some knowledge of the human world- learning latin is kinda a safe bet. Thousands of years ago you could find plenty of folks that knew it, today people speak it thousands of years later- Sumerian, try and find someone in the last 2,000 years fluent in Sumerian. It’s not as common past or present in near as wide a reach as Latin. You can find SOMEONE on every continent that can speak latin I’m sure. In more recent history English is a good one to know- “Chinese” would be good or “Indian” but there aren’t singular languages that cover ALL people in those groups. In Indian and Pakistan, throughout China and its territories are many dialects that aren’t all understood by even most of the speakers. A few are more dominant though and still likely good languages of you want the most people who understand.

There are other theories too though- that latin- being at its roots a somewhat “holy language” is just “fun” for demons and followers to profane through its use for example. Or the opposite of the “truth and precision” theory- latin is very precise but it can also be difficult and full of possible word play- so demons might like laying because if they are more fluent and capable in latin they can easily skip things in unnoticed to the novice speaker. They can rely on the novice speaker to foul up a pronunciation or a sentence structure in such a way that is favorable to them.

In fact- that precision of language could make it favorable to translate and use Latin over original texts. Think about it like this- if the conjuration of Magic or summoning and bonding of a demon requires EXACT phrasing or concepts and pronunciation and syntax- latin may allow for direct translation from earlier forms, whereas much like how the Bible has MANY slightly different versions that vary by as little as using “you” instead of “thou” or so forth- that TINY difference could make a spell useless or dangerous.

This is fascinating... I love facts like this. My favourite pastime

I’m glad you enjoyed it! It’s somewhat fitting to your user name too I suppose.

Commenting so I can read later after my quiz

This was interesting and fun. I shall put it to serious consideration for all future works of fiction both intaken and created by me.